Heretofore, it has been common practice to use poppet-type intake and exhaust valves for the cylinders of conventional internal combustion engines to control the passage or flow of gases into and out of the cylinders. These valves are usually spring loaded toward a valve-closed position and are biased open either by a cam and push rod mechanism or by a direct-acting overhead cam mechanism. In either mechanism, the cam shaft is connected to and rotates in synchronization with the engine crankshaft to facilitate the opening and closing of each valve at timed intervals in the engine cycle. Since the valves are linked to the crankshaft, the lift distance is fixed and the lift rate or speed of the closure member of each valve is dependent on and proportional to the engine or crankshaft speed. This restriction, which is well known in the industry, limits engine performance relating to fuel consumption, emissions, torque or output, and idle quality. The overall durability of the engine is also affected because the seating velocity of the valve is dependent on the engine speed.
To eliminate these deficiencies in engine performance and durability, variable valve actuation has been developed to vary the lift distance, lift speed, and seating velocity of the intake and exhaust valve closure members. Variable valve actuation generally enables adjustment of the valve motion profile to meet the ever-changing conditions and demands placed on the engine. Numerous types of variable valve actuator systems have been proposed, some of which are set forth in the 1989 article entitled A Review and Classification of Variable Valve Timing Mechanisms by Thomas Dresner and Philip Barkan (Society of Automotive Engineers, Inc. Paper 890674). It is clear from this article and the prior art that tremendous advantages can be obtained by substituting an independent variable valve actuating system for the conventional cam actuated intake and exhaust valve systems.
These advantages generally include conserving fuel, protecting the environment, and increasing engine output and durability. For instance, it is generally agreed that electromechanical variable valve actuation will increase overall valve train efficiency by eliminating the frictional losses of the cam mechanism, the weight of the cam mechanism, and the cam mechanism's consumption or drain of power from the crankshaft. A further advantage of the variable valve actuating system is that the seating velocity of the valve closure member could be reduced to lessen wear on both the valve seat and valve head, thereby increasing the overall life of the valve and the engine. Another benefit of variable valve actuation is the possibility of creating a variable-cycle engine wherein certain valves would remain closed at certain engine speeds to allow the engine to operate as a two-cycle engine at those speeds. Yet another benefit of a variable valve actuating system is that the valve profile could vary to control engine load without use of the throttle, thereby decreasing pumping losses. Variable valve actuation may also enable the engine to be a multi-fuel engine. These advantages increase the engine's efficiency and output while lowering the amount of pollutants emitted.
As explained in the article cited above, different types of variable valve actuator systems have been designed to solve these problems, including substantial work directed toward the use of solenoids and toward the use of magnetic attraction/repulsion principles for opening and closing intake and exhaust valves. These type of systems are exemplified by the following patents: U.S. Pat. Nos. 3,853,102; 3,882,833; 4,762,095; 4,794,890; 4,829,947; 4,831,973; 4,841,923; 4,846,120; 4,942,851; 4,984,541; 5,009,202; 5,009,389; 5,070,826; 5,076,221; and 5,197,428.
While solenoid and magnetic attraction/repulsion valve actuator systems solve some of the problems associated with cam actuated valve systems, a host of additional problems arise in such valve actuator systems. These additional problems have prevented the widespread acceptance and use of these types of variable valve actuators in production internal combustion engines even though it is widely agreed that variable valve actuating systems will dramatically increase engine performance, efficiency, and durability while decreasing pollution.
The solenoid and magnetic attraction/repulsion actuating devices generally employ an iron or ferromagnetic armature which limits the performance of the actuator because it requires a variable air gap to generate force. As the air gap becomes larger-when the distance between the moving and stationary magnets or electromagnets increases, there is a reduction in the force applied to the armature. To maintain high forces on the armature as the size of the air gap increases, a higher current is employed in the coils of such devices. This increased current leads to higher energy losses in the system and the possibility of overheating of the coils. The non-constant force profile also makes the precise control of the valve more complex, requiring additional control mechanisms to control the valve closure member's lift distance and lift speed. The result of this is that most current designs have high seating velocities and do not vary the valve lift.
Another problem relating to variable valve actuating devices presently known is that efficiency is hampered by hysteresis losses. Hysteresis losses are caused by the termination or reversal of a magnetic field in iron. These losses are associated with the solenoid actuators of the prior art.
Accordingly, while it is generally agreed that variable valve actuator systems will greatly increase internal combustion engine efficiency, durability, and output, no efficient and inexpensive electromechanical valve actuating system has been generally acceptable for widespread use in production internal combustion engines.